Mine detector with NQR-SQUID

ABSTRACT

A portable small mine detecting device for detecting a mine by transmitting a radio wave and detecting the NQR of nitrogen 14 atoms (14N) contained in the mine using a high-sensitivity, high-temperature superconducting SQUID magnetic sensor. The mine detecting device can be applied to non-metal mines and can detect different kinds of explosive substances simultaneously.

TECHNICAL FIELD

The invention of this application relates to a mine detecting deviceand, more particularly, to a mine detecting device for detecting a mineby detecting the NQR (Nuclear Quadrupole Resonance) of nitrogen 14 atomsof the explosive substance contained in the mine.

BACKGROUND ART

A number of Patent Publications on the technique of detection/removal ofa mine have been applied not only in Japan but also in foreigncountries, for instance References 1 to 4. However, not only thetechniques described in the above-specified publications but also thetechniques as described in many publications thus far published fordetecting/removing a mine are detection methods for recognizing the mineas a foreign substance from the soil around it, in which the mine isburied, by employing an underground radar or a metal detector.

Publication 1: JP-A-2001-74387,

Publication 2: JP-A-2001-153597,

Publication 3: JP-A-06-506295, and

Publication 4: U.S. Pat. No. 6,411,208B1.

The mine detecting method thus far developed is roughly divided into twokinds. One is a method detecting the difference in physical propertiesbetween the mine and the soil around that, in which the mine is buried,and the other is a method for detecting the physical properties of themine (or the explosive substance) itself directly.

The methods classified among the former include an electromagneticinduction method (metal detection), an electric wave method (dielectricrate), an electric survey method (specific resistance), a thermal surveymethod (heat capacity), an ultrasonic method (substance density), and soon. The methods classified among the latter include a nuclear magneticresonance method (magnetic characteristics), a neutron method(radioactivation characteristics), a chemical method (bonding state ofatoms), a biological method (with an antibody bio-film) and so on.

The nuclear magnetic resonance NMR method classified among the lattergenerally uses a nuclear magnetic resonance spectrometer, as is utilizedat present in medical devices.

This NMR method, detecting a mine by means of the nuclear magneticresonance, does not detect the differences in the physical propertiessuch as the electric conductivity or the dielectric constant between themine and the soil in which the mine is buried, as does theelectromagnetic induction method (metal detector) or the electric wavemethod (underground radar). The NMR method directly detects theintrinsic nuclear magnetic field of the atoms composing the mine (or theexplosive substance). The NMR method is accepted as excellent minedetecting means because it can detect the explosive substance of thedetection target directly.

For the NMR method, however, a large-sized magnet is indispensable forgenerating an intense magnetic field. Thus, this NMR method utilizingnuclear magnetic resonance has a fatal defect when it is desired toreduce the size of the device. Therefore, the chemical substancedetecting device according to the NMR method has been limited inpractice to the MRI (Magnetic Resonance Imaging) for medical diagnosis,because the NMR method is viewed as not suited for a portable minedetecting device.

Therefore, the invention described in this application is presented tosolve the problems identified in the prior art and to provide a minedetecting device, which is made so portable that it can be convenientlyemployed even outdoors.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problems, according to theinvention of this application, there is firstly provided a minedetecting device comprising an electromagnetic wave transmitter; anelectromagnetic wave transmitting antenna; and a high-temperaturesuperconducting quantum interference device SQUID for receiving the NQRsignal of nitrogen 14 atoms contained in an explosive substance. Theinvention provides secondly a mine detecting device comprising anenvironmental magnetic field receiving SQUID; thirdly a mine detectingdevice characterized in that the cooling medium of the high-temperaturesuperconducting SQUID and/or the environmental magnetic field receivingSQUID is liquid nitrogen; and fourthly a mine detecting devicecharacterized in that the high-temperature superconducting SQUID and theenvironmental magnetic field receiving SQUID are connected with adifferential circuit.

Moreover, the invention of this application provides fifthly a minedetecting device characterized by using a receiving coil connecting afirst order differential or second order differential pickup coil of anormal conducting metal wire and an input coil for introducing themagnetic field into the SQUID housed in a magnetic shield; sixthly, aportable mine detecting device characterized in that the electromagneticwave transmitting antenna and the high-temperature superconducting SQUIDare arranged so that they can be gripped. Additionally, theelectromagnetic wave oscillator, a high-temperature superconductingSQUID controller and a data processor can be driven by a battery.Seventhly, a mine detecting device, wherein the frequency of thetransmitted electromagnetic wave is a radio wave in the band from 0.1 to10 MHz; eighthly, a mine detecting device characterized by sweeping thetransmitted electromagnetic wave over the 0.1 to 10 MHz band andutilizing the resonance signals of the NQR signal obtained; ninthly, amine detecting device characterized in that the electromagnetic wavetransmitting antenna has a directivity; and tenthly, a mine detectingdevice wherein a square wave is transmitted from the electromagneticwave transmitting antenna, and the frequency spectrum obtained by thequick Fourier analysis of the signal detected by the high-temperaturesuperconducting SQUID is compared with the spectral distributions ofchemical substances from a database.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the entire mine detecting device.

FIG. 2 is a diagram of the entire mine detecting device employing amagnetic shield.

FIG. 3 is a schematic diagram showing the concept of the mine detectingdevice.

FIG. 4 shows the concept of a mine detection utilizing NQR.

FIG. 5 is a diagram showing the relation between frequency andsensitivity.

Here, reference numerals in the drawings designate the followingcomponents.

1 Radio Wave Transmitting Antenna 2 Radio Wave Transmitter 3 Lock-InAmplifier 4 Ground 5 Liquid Nitrogen 6 Liquid Nitrogen Container 7SQUID1 8 SQUID2 9 Electronic Circuit for SQUID1 10 Electronic Circuitfor SQUID2 11 Differential Amplifier 12 Amplifier 13 Mine 15 Secondorder Differential Type Coil 16 Magnetic Shield 17 Magnetic InductionCoil

DETAIL DESCRIPTION OF THE INVENTION

According to the principle of the invention of this application, anexplosive substance to be detected such as TNT (Trinitrotoluene)employed as mine explosive is detected by transmitting an ACelectromagnetic field into the ground, causing the nuclear quadrupoleresonance (abbreviated as “NQR”) of nitrogen 14 atoms (14N) to generateelectromagnetic radio waves intrinsic to these atoms. The invention ischaracterized by utilizing a superhigh-sensitivity magnetic sensorcalled a superconducting quantum interference device (as will beabbreviated into the “SQUID”), for detecting the low-frequency band,which has been especially difficult to detect.

The concept of the mine detecting device combining the NQR signal andthe SQUID is schematically shown in FIG. 3. A radio wave is transmittedfrom a radio wave transmitter through a radio wave transmitting antenna.The explosive substance, contained in a mine buried in the ground, isidentified because when the mine is irradiated with the radio wave themine resonates with the NQR and this signal is received by the SQUID,and by contrasting the NQR with known resonance frequencies by means ofa data processor utilizing a personal computer or the like. The NQRemployed in the mine detecting device of the invention of thisapplication detects the substance on a principle like that of the NMR(Nuclear Magnetic Resonance Spectrometer). The essential differencebetween the NQR and the NMR is that the NMR rises in magnetic fields,whereas the NQR rises in an electric field gradient around an atomicnucleus so that it is advantageous for specifying a substance in a zeromagnetic field.

The principle of the NQR is shown in FIG. 4. As shown in the schematicdiagram of FIG. 4, an atom having nuclear spin of one or more is made tohave a resonance frequency intrinsic to the atom by the interactionbetween the electric field gradient and the nuclear quadrupoles aroundthe atomic nucleus so that the substance can be identified from thatresonance frequency. Nowadays, the resonance frequencies intrinsic toseveral hundreds of thousands of chemical substances have already beenexamined to make it easy to detect the target explosive substance.

For example, most explosive substances are those containing nitrogensuch as the TNT. This makes it possible to catch the NQR signal from thenitrogen 14 (¹⁴N) having each spin of 1.

The electric waves to be usually employed for detecting the NQR signalare radio waves in the range of 10 MHz or less. Usually, the target isdetected by bringing an antenna, for irradiating the electric wave ofthat range, closer to the target. However a remote explosive substancecan be detected by using an electric wave transmitting antenna withdirectionality.

However, the resonance frequency caused by the NQR signal is generallyonly a few MHz (Megahertz), which is much lower than that of theordinary NMR. This raises a problem that the electromagnetic wavedetecting coil usually employed cannot not detect the explosivesubstance sufficiently.

Relations between that frequency (f) and the sensitivity are shown inFIG. 5. It is apparent from FIG. 5 that the reception sensitivity of theNQR signal by the electromagnetic wave detecting coil is seriouslylowered in the low-frequency band but that the SQUID sensitivity isconstant, independent of change in the frequency (f).

The invention of this application is contemplated to eliminate thedefect of the mine detecting device utilizing the NQR signal, by usingthe SQUID as a detector in the cases where the resonance frequency hasbeen too low to be detected sufficiently.

This SQUID is a high-sensitivity magnetic sensor utilizingsuperconducting quantization, and has a sensitivity one hundred times ormore higher than that of the magnetic sensor of the prior art. Thus, itcan detect a weak magnetic field one fifty millionth or less of theearth's magnetic field.

In the invention of this application, it is necessary to employ not thegeneral SQUID using helium as a cooling medium but rather ahigh-temperature superconducting SQUID. This is because the SQUID of theprior art employing liquid helium as the cooling medium not only hard tohandle but also has difficulties related to the high cost for the liquidhelium and the large size of the heat insulation. Thus, it is thoughtdifficult to utilize the SQUID for the portable mine detecting device.

On the other hand, the high-temperature superconducting SQUID is easy tohandle and can employ the liquid nitrogen (at 77.3 K: −196° C.) at a lowcost so that it is small and light. Moreover, the mine detecting devicecan be made portable by utilizing the high-temperature superconductingSQUID in the mine detecting device. In the invention of thisapplication, therefore, the SQUID implies the high-temperaturesuperconducting SQUID.

However, the superhigh-sensitivity magnetic sensor utilizing the SQUIDis so extremely sensitive as to invite a problem that the mine detectingdevice in actual use may pick up noise. In order that the noise may beeliminated to allow detection of the presence of the explosive substanceprecisely, an environmental noise measuring SQUID has to be connected toeliminate the noise. This mode is shown in FIG. 3. For this noiseelimination, this mine detecting device is provided with not only thesignal receiving SQUID but also the magnetic noise receiving SQUID, andthus detects only the NQR signal from the explosive substance.

Here, the signal receiving SQUID or the magnetic noise receiving SQUIDcan be cooled down merely by dipping it in a liquid nitrogen containerfilled with the liquid nitrogen.

Then, the environmental noises are removed by connecting the signalreceiving SQUID and the magnetic noise receiving SQUID with adifferential circuit. The signals thus cleared of the environmentalnoises are processed by a data processor such as a personal computer.

Here, the mine detecting device of the invention of this application ischaracterized in that it is enabled to identify and detect a pluralityof substances simultaneously by changing the frequency. The band of thefrequency at this time should not be especially limited but preferablyfalls within the range of 0.1 to 10 MHz.

As has been detailed hereinbefore, the mine detecting device of theinvention of this application has many features distinguishing it fromother mine detecting devices. The excellent portions of the invention ofthis application may be enumerated as follows.

(a) The device can detect the explosive substance itself directly.

(b) The device can detect a plurality of different explosives (or mines)simultaneously by changing the frequency.

(c) The device can be made small and portable.

(d) The device needs no magnetic field for the detection.

(e) The device can perform a high-sensitivity measurement by using theSQUID as the sensor.

(f) The device can operate with just a small quantity of liquid nitrogenby utilizing the high-temperature superconducting SQUID.

The invention of this application has the features described above, andis described in detail in connection with its modes of embodimentutilizing that device.

EMBODIMENTS Embodiment 1

FIG. 1 is an entire diagram showing a mine detecting device as anembodiment. A radio wave (0.1 to 10 MHz) for the detection istransmitted from a radio wave transmitter (2) and amplified by anamplifier (12). The amplified radio wave is then introduced into a radiowave transmitting antenna (1) so that the radio wave is transmittedtoward the ground surface.

A resonant NQR signal is transmitted from a mine (13) having receivedthat radio wave and is received by a signal receiving SQUID1 (8). AnSQUID2 (7) is provided for measuring the environmental magnetic noises.As a result, signals passing through the individual electronic circuits(10) and (9) of the SQUID1 (8) and the SQUID2 (7) are outputted to adifferential amplifier (11) where the NQR signal is cleared of theenvironmental noises and only the NQR signal is introduced into alock-in amplifier (3).

The lock-in amplifier (3) takes in the signal of the radio wavetransmitter (2) as a reference signal so that it can extract only thesignals introduced into the differential amplifier (11) that have thefrequency of the radio wave, thereby reducing the noise. That signal issubjected to an A/D conversion and inputted to a processor utilizing apersonal computer (PC) or the like. This signals are integrated andaveraged 100 to 10,000 times, so that the final data has reducedenvironmental noise. These operations are repeated at a frequency of 10Hz while varying the radio frequency from 0.1 to 10 MHz so that thespectrum of the entire range of 0.1 to 10 MHz can be sampled.

Additionally, for simplification, the efficiency of the mine detectioncan be raised by performing the operations thus far described for only afrequency known to be significant (see Table 1).

TABLE 1 NQR Spectrum of Typical Explosives Unit (MHz) TNT RDX HMXNitrotoluene Trinitrotoluene Hexogen Octogen p-nitrotoluenem-nitrotoluene C₇H₅N₃O₆ C₃H₆N₆O₆ C₄H₈N₈O₈ p-C₇H₇NO₂ m-C₇H₇NO₂ 0.871 5.245.306 1.198 1.19 0.8604 5.192 5.068 0.911 0.91 0.845 5.047 3.737 0.94383.458 3.625 0.838 3.41 1.564 0.769 3.359 1.441 0.752 1.782 0.743 1.6880.716

SOURCE

LANDOLT-BÖRNSTEIN

Vol. 20

Nuclear Quadrupole Resonance Spectroscopy Data

Editors: K.-H. Hellwege and A. M. Hellwege

Springer-Verlag Berlin Heidelberg 1988

Specifically, a search for TNT, nitrotoluene, RDX and HMX was executed.The result was that the NQR signal for all the explosives in amountsdown to the detection limit of the SQUID could be clearly confirmed.Below is given a detailed description of the search for TNT which isfrequently employed in mines.

100 g of TNT was buried in the ground (4), and the NQR signal wasdetected by means of a SQUID 5 cm away. The NQR signal had an intensityof about 1 pT (picoteslas). The SQUID employed in this method had asuperconducting film of Y₁Ba₂Cu₃O₇ having a thickness of 0.1 microns andformed on the substrate of a square of 10 mm. For the measurements, theelectronic circuit of FIG. 1 was used, and the output of the lock-inamplifier was received and monitored by the processor utilizing thepersonal computer (PC) or the like. The integration averaging operationswere done 1,000 times. This SQUID has a magnetic resolution of about0.01 pT (picoteslas) so that the NQR signal of the TNT could be clearlyreceived.

Embodiment 2

FIG. 2 is a diagram of the entire mine detecting device, which canmeasure with minimum influence of environmental noise. In this minedetecting device, the NQR signal is caught by a second orderdifferential coil (15), its magnetic field is taken in by a magneticinduction coil (17) and outputted as a SQUID magnetic field, and this isdetected by the SQUID.

The second order differential coil (15) has 100 upper turns, 200 middleturns and 100 lower turns and a diameter of 5 cm. The upper and lowercoils are turned in the common direction, but the middle coil is turnedin the reverse direction. Moreover, the magnetic induction coil (17) has200 turns and a diameter of 5 cm. The SQUID (8) dipped in liquidnitrogen (5) is disposed just under the magnetic induction coil (17),thereby to monitor the magnetic field. The magnetic induction coil (17)and the SQUID (8) are disposed in a magnetic shield (16). This magneticshield (16) has a double structure, as shown in FIG. 2, and is made ofPermalloy of a high magnetic permeability. This magnetic shield (16)made of a double cylinder (having a bottom and an upper cover) preventsany environmental magnetic field from being applied to the SQUID. Ithas, therefore, been confirmed that the SQUID (8) worked stably. Thesame method as that of the embodiment was used for the experiments todetect the explosives. The NQR signal were about 1 pT (picotesla)whereas the SQUID noises were 0.01 pT (picoteslas), thus confirming thatthe NQR signal had sufficient strength for the measurements.

The mine detectors of the prior arts are mostly metal detectors.However, recent mines have increasingly been non-metallic ones such asplastic bombs. The mine detecting device of the invention of thisapplication can be applied to such mines of plastics and can also bemade portable. Thus, this mine detecting device can be expected to beused widely as the mine detector in the future.

1. A portable mine detecting device comprising: an electromagnetic wavetransmitter for generating an electromagnetic wave; an electromagneticwave transmitting antenna connected to said electromagnetic wavetransmitter and being configured to transmit the electromagnetic wave ina direction of an explosive substance; and a high-temperaturesuperconducting quantum interference device (SQUID) for receiving andprocessing a nuclear quadrupole resonance (NQR) signal from a pluralityof nitrogen 14 atoms contained in the explosive substance, wherein theNQR signal is generated as a result of the plurality of nitrogen 14atoms in the explosive substance being irradiated by the transmittedelectromagnetic wave.
 2. The portable mine detecting device of claim 1,further comprising an environmental magnetic field receiving SQUID. 3.The portable mine detecting device of claim 2, further comprising acooling medium configured to cool at least one of said high-temperaturesuperconducting SQUID and said environmental magnetic field receivingSQUID, said cooling medium being liquid nitrogen.
 4. The portable minedetecting device of claim 3, further comprising a differential circuitconnected to said high-temperature superconducting SQUID and saidenvironmental magnetic field receiving SQUID.
 5. The portable minedetecting device of claim 3, further comprising a receiving coiloperable to be connected to a first order differential pickup coil or asecond order differential pickup coil comprised of a conducting metalwire, and an input coil for introducing a magnetic field into saidhigh-temperature superconducting SQUID housed in a magnetic shield. 6.The portable mine detecting device of claim 2, wherein saidelectromagnetic wave transmitting antenna and said high-temperaturesuperconducting SQUID are configured to be held; and saidelectromagnetic wave transmitter, said high-temperature superconductingSQUID, and a data processor are configured to be powered by a battery.7. The portable mine detecting device of claim 2, wherein thetransmitted electromagnetic wave has a radio wave frequency band of 0.1to 10 MHz.
 8. The portable mine detecting device of claim 1, furthercomprising a cooling medium configured to cool said high-temperaturesuperconducting SQUID, said cooling medium being liquid nitrogen.
 9. Theportable mine detecting device of claim 8, further comprising adifferential circuit connected to said high-temperature superconductingSQUID and another SQUID.
 10. The portable mine detecting device of claim9, wherein said electromagnetic wave transmitting antenna and saidhigh-temperature superconducting SQUID are configured to be held; andsaid electromagnetic wave transmitter, said high-temperaturesuperconducting SQUID, and a data processor are configured to be poweredby a battery.
 11. The portable mine detecting device of claim 9, whereinthe transmitted electromagnetic wave has a radio wave frequency band of0.1 to 10 MHz.
 12. The portable mine detecting device of claim 8,further comprising a receiving coil connected to a first orderdifferential pickup coil or a second order differential pickup coilcomprised of a conducting metal wire, and an input coil for introducinga magnetic field into said high-temperature superconducting SQUID housedin a magnetic shield.
 13. The portable mine detecting device of claim12, said electromagnetic wave transmitting antenna and saidhigh-temperature superconducting SQUID configured to be held; and saidelectromagnetic wave transmitter, said high-temperature superconductingSQUID, and a data processor are configured to be powered by a battery.14. The portable mine detecting device of claim 12, wherein thetransmitted electromagnetic wave has a radio wave frequency band of 0.1to 10 MHz.
 15. The portable mine detecting device of claim 8, whereinsaid electromagnetic wave transmitting antenna and said high-temperaturesuperconducting SQUID are configured to be held; and saidelectromagnetic wave transmitter, said high-temperature superconductingSQUID, and a data processor are configured to be powered by a battery.16. The portable mine detecting device of claim 8, wherein thetransmitted electromagnetic wave has a radio wave frequency band of 0.1to 10 MHz.
 17. The portable mine detecting device of claim 1, whereinsaid electromagnetic wave transmitting antenna and said high-temperaturesuperconducting SQUID are configured to be held; and saidelectromagnetic wave transmitter, said high-temperature superconductingSQUID, and a data processor are configured to be powered by a battery.18. The portable mine detecting device of claim 1, wherein thetransmitted electromagnetic wave has a radio wave frequency band of 0.1to 10 MHz.
 19. The portable mine detecting device of claim 1, whereinthe transmitted electromagnetic wave is transmitted toward the explosivesubstance by varying the transmitted electromagnetic wave between a 0.1to 10 MHz band, and the NQR signal from the explosive substance isobtained as a result of the transmitted electromagnetic wave.
 20. Theportable mine detecting device of claim 1, wherein a square wave istransmitted from said electromagnetic wave transmitting antenna so thata frequency spectrum obtained by a quick Fourier analysis of the NQRsignal detected by said high-temperature superconducting SQUID iscompared with spectral distributions of chemical substances contained ina database.